1. Field of the Invention
The present invention relates to processes for utilizing industrial wastes and industrial process byproducts to help reduce emissions of undesirable compounds, improve operating economics, and to provide useful chemicals from such wastes and byproducts. More particularly, the present invention relates to processes for the practical utilization of industrial wastes/process byproducts, such as those resulting from the operation of electrical power generating plants, and also to enhance power generating plant operability. The processes of this invention help to reduce environmental damage from byproducts of the operation of such plants, and they also can be utilized for producing marketable chemicals from such byproducts.
2. Description of the Related Art
In fossil-fuel-fired power generating plants, as well as in other industrial processes involving combustion of fossil fuels, the products of the combustion process include compounds that are environmentally undesirable, and the discharge of which into the environment is often subject to environmental regulations. Such compounds include sulfur oxides (SOx) and nitrogen oxides (NOx). In order to meet environmental limitations affecting the discharge of SOx into the atmosphere, combustion products from such plants and processes are commonly passed through flue gas desulfurization (FGD) systems to block their discharge into the atmosphere by converting the sulfur oxides contained in such combustion products into less-environmentally-harmful insoluble compounds that are either disposed of in landfills or that, when suitably modified or treated, are sold as marketable chemicals
The treatment of flue gases is often effected in lime- or limestone-based wet scrubbers, in which lime or limestone slurries are sprayed into the flue gases before they are discharged into the atmosphere. The sulfur oxides present in the flue gases are thereby chemically converted into insoluble calcium compounds in the form of calcium sulfites or sulfates. Those calcium compounds, which are less environmentally harmful, are separated from the scrubber blowdown liquor and either are disposed of in landfills or are converted into marketable gypsum.
Because the scrubber blowdown liquor is collected within the scrubber, withdrawn, and recycled by reintroduction into the scrubber to continuously treat the flue gases, soluble impurities that form during the scrubbing process and that remain in the blowdown liquor tend to increase in concentration as the liquor is reused after separation of the calcium compounds. The impurities, derived largely from the lime or limestone, tend to be primarily magnesium salts (sulfate, bisulfite, and chloride), with lesser amounts of sodium, potassium, and calcium compounds. The concentrations of those soluble impurities will vary, based upon particular process operating practices and conditions. The amount of total dissolved solids present in the blowdown liquor is usually controlled by the bleed or withdrawal rate and can ultimately rise to be of the order of about 6% or 7%, after which treatment of the liquor to remove dissolved solids should be undertaken. Discharge of such dissolved solids to local streams or lakes is generally environmentally unacceptable, and thus there is a need for an acceptable way to reuse or to dispose of the scrubber blowdown liquors.
Although useful for converting some sulfur oxides, widely-used lime/limestone scrubbers are not very effective in capturing the 1% to 1.5% of the sulfur in the fuel that is transformed during the combustion process into gaseous SO3, which can escape from the scrubber. The SO3 also poses operating problems within the furnace itself, in that it leads to corrosion and fouling of low temperature heat exchange surfaces. Additionally, it poses environmental problems in that unless captured or transformed the SO3 in the flue gas results in a persistent, visible stack discharge plume, which can contain corrosive and potentially hazardous sulfuric acid fumes. Further complicating the matter, selective catalytic reactors, which are available and are generally being_installed in such plants to comply with nitrogen oxide emission regulations, essentially cause a doubling of the amount of SO3 that is generated, and consequently the already serious operational and environmental problems caused by the presence of SO3 are magnified. In the past, the SO3 emission problem has been addressed chemically using commercially-available, but relatively expensive, oil-based magnesium additives.
One of the more effective chemical techniques for controlling both ash-related fouling in the furnace, and also the corrosion and emission problems associated with SO3 generated in coal-fired furnaces, is the injection into the upper region of the furnace of oil slurries of MgO or Mg(OH)2. That technology was originally developed for use with oil-fired furnaces, in which the magnesium additives were usually metered into the fuel. It was later applied to coal-fired furnaces, in which the most widely accepted mode of application of such additives today has been by injection of slurries of MgO or Mg(OH)2 into the furnace above the burners and just below the region at which a transition from radiant heat transfer to convective heat transfer occurs.
In addition to the oil-based slurries, magnesium-containing powders and water-based Mg(OH)2 slurries have also been utilized as fireside additives in boilers, but because of their generally coarser particle size they are less efficient in capturing the SO3. Water slurries of MgO have been injected through specially modified soot blowers on oil- and Kraft-liquor-fired boilers, where they moderated high temperature deposit problems but had only nominal impact on SO3-related problems because of an inability to apply the chemicals on a continuous basis, resulting from the intermittent, rather than continuous, operation of the soot blowers.
Although the powders and the water-based slurries containing magnesium that have been utilized in the past have had some inhibiting effect on SO3 formation, the overall efficiency of utilization as an SO3 treatment vehicle is relatively poor. The lower efficiency of utilization therefore requires that higher quantities of such additives be injected. However, such higher quantities of the injected materials can adversely impact the marketability of the coal fly ash that is collected in the precipitator, because the higher additive quantities increase the proportion of undesirable MgSO4 in the fly ash. Both magnesium and sulfate can be tolerated in marketable fly ash, but only at modest levels.
Industry practice in efforts to reduce ash-related fouling have primarily involved the introduction of the oil- or water-based slurries into the upper region of the furnace at locations where the temperatures are high enough to decompose the materials to provide the active ingredient MgO. But it does not appear that there has been employed a combination of application techniques that are designed to achieve maximum efficiency by treating each fireside problem—ash deposits, corrosion, and emissions—with a chemical form and an application mode that are best suited to solve a particular problem
Introduction of previous magnesium additives into the upper, high temperature region of the furnace for SO3 inhibition has been only partially effective. Attempts have been made to enhance performance efficiency both by injecting oil- or water-based slurries in the high temperature region of the furnace and by supplementing that hot section additive with a cold end additive at the cold section of the plant, generally after the precipitator. The cold end additive is intended to neutralize SO3 that is not captured by the additives injected into the higher temperature sections of the furnace. Generally, the cold end additive is an MgO powder that is blended with a flow-improving agent to mitigate the difficult problems often encountered in handling and metering relatively fine solids. Although use of a cold end additive avoids potentially adverse impacts on precipitator performance, the stoichiometric excess required can be as much as one order of magnitude higher than with furnace injection. There is thus a need for a more effective treatment approach.
Byproducts produced in the course of power generation and in the course of controlling and minimizing adverse environmental consequences of power generation plant operation are often considered not to be commercially useful. For example, in fossil-fueled power generating plants in which magnesium oxide is utilized in gas desulfurization systems, significant amounts of mixed magnesium sulfites and magnesium sulfates are produced as a byproduct and are discarded in landfills. Typical concentrations in such byproducts are about 80% magnesium sulfite and the balance magnesium sulfate.
Insoluble calcium sulfites or calcium sulfates that are produced in lime or limestone scrubbers utilized in flue gas desulfurization systems are often disposed of in landfills after separation from the scrubber liquor. Such systems are utilized in power generation plants or in industrial process plants in which high-sulfur fossil fuels are burned, and they generate large quantities of those compounds as they convert the undesirable and environmentally-regulated emissions of sulfur dioxide into the calcium sulfites or sulfates. In an effort to avoid landfill costs, more and more of those compounds are being converted to marketable gypsum by oxidation in the scrubber or in a separate vessel.
And after separation of the sulfites and sulfates from the limestone scrubbers and recycling of the blowdown stream, the resultant scrubber liquor contains soluble impurities that tend to increase in concentration over time with continued recycling. Such impurities, which are derived largely from the limestone, tend to be primarily magnesium salts (sulfate, bisulfite, and chloride), with lesser amounts of calcium, sodium, and potassium compounds. The concentrations can vary with operating practice, and can be of the order of about 6 to 7%. Although those salts are not particularly toxic, discharge into local streams or lakes is often environmentally unacceptable, even after treatment.
In the limestone industry significant quantities of fine limestone particles are produced in the course of crushing and screening operations. Such particles are of the order of −60 mesh and finer, and are either calcium carbonates or calcium and magnesium carbonates. Because such fine particles generally have not passed through a calciner, they retain much of the carbon dioxide that was present in the limestone. However, they have little or no commercial value in and of themselves and are therefore usually discarded.
In some metallurgical and organic chemical processes, including titanium production, significant amounts of hydrochloric and sulfuric acids are produced as byproducts, neutralized, and the resulting soluble salts discharged to the environment. The concentrations and impurity levels and constituents vary with the acid and the source. Weaker concentrations of hydrochloric acids, of the order of about 18 to 28%, are considered to be waste products because commercial grades of HCl are typically concentrations of 32% or more. With respect to sulfuric acids, commercially-significant concentrations are normally 98% or more, whereas byproduct acids generally have concentrations of 93% or less and include organic contaminants.
In steelmaking furnaces and cement kilns, used refractory brick are generally discarded. A significant fraction of such used brick are of the magnesitic or dolomitic variety and are initially relatively pure (98%) magnesium oxide and a mixture of calcium and magnesium oxide. The bricks are generally formed using pitch or other bonding agents, which along with iron from the molten steel after use in furnaces result in impurities that lower the oxide concentrations in used bricks to within the range of about 77% to about 93%.
It is therefore an object of the present invention to provide processes for treating fossil-fuel-fired furnace fireside and emissions problems.
It is another object of the present invention to provide processes by which otherwise-disposed-of byproducts of industrial processes can be effectively utilized, either as more effective furnace additives or as separately marketable chemical compounds.